docs: def _pick_up_classes(modules, classnames):
      """ Given a list of class names to retrieve, try to fetch them from the specified list of
          modules and returns the list of the fetched classes.
      """
      klasses = []
      for classname in classnames:
          klass = None
          for module in modules:
              if hasattr(module, classname):
                  klass = getattr(module, classname)
                  break
          if not klass:
              raise ClassNotFoundError('Error fetching \'{}\' in {}'.format(
                  classname, str([module.__name__ for module in modules]))
              )
          klasses.append(klass)
      return klasses